Hepatitis C virus (HCV) appears to be the major current etiological agent of transfusion-related non-A, non-B hepatitis. The HCV genome is a linear, positive-strand RNA molecule of approximately 9,500 nucleotides and encodes a polyprotein of about 3,000 amino acids. Several stretches of amino acids in the HCV polyprotein share significant similarity with flavivirus and pestivirus proteins. Therefore, HCV is considered to be distantly related to these virus groups. The goal of this project is to increase our understanding of the molecular biology of this important human pathogen. Extensive sequence analysis has been performed on the 5' noncoding (NC) region, core, envelope 1 and hypervariable region (HVR1) of envelope 2 genes of over forty strains of HCV. In addition, neutralization assays, based upon blocking of attachment of virus to susceptible cells in vitro and prevention of infection of chimpanzees in vivo, have been developed and are being characterized for specificity and sensitivity. Prototype strains of the various genotypes of HCV, including some of those discovered in this laboratory, are being biologically amplified in chimpanzees and further characterized. Pools of virus-containing plasma will be packaged and distributed for further characterization, and used as challenge inocula in studies of passive and active immunoprophylaxis, etc.